As taught in U.S. Pat. No. 4,748,275 (Smith et al.) and the references discussed therein, there are many known methods of reacting mixed tert-amines with aqueous hydrogen peroxide to form tert-amine oxides including at least one long-chain group and at least one short-chain group.
The syntheses most commonly employed for the preparation of these oxides are the aqueous processes utilizing sufficient water to provide the products as aqueous solutions, including the processes of U.S. Pat. No. 4,247,480 (Murata et al.) in which carbon dioxide is used to promote the reaction. The aqueous processes are quite satisfactory for the products which are to be used in applications in which their water content can be tolerated. However, the utilization of these processes necessitates the performance of after-treatments, such as spray-drying or evaporation, when the amine oxides are intended for use in applications, such as dry solid laundry detergent formulations, in which the presence of the solvent cannot be tolerated.
Less commonly, the tert-amine oxides are prepared in organic solvents, as in U.S. Pat. No. 3,776,959 (Stalioraitis et al.). These processes are also very satisfactory for some purposes, but their being designed to form the amine oxides in solution makes them present the same type of problem as the aqueous processes when they are employed to prepare amine oxides that are intended for use in applications in which the presence of a solvent cannot be tolerated. After-treatments are required to remove the solvent.
Smith et al. teach that some tert-amine oxides can be prepared in the solid form that makes them more desirable than the dissolved oxides for some purposes. Their process uses temperatures high enough to maintain the product in a molten state.
Some tert-amine oxides have been synthesized as dihydrates; and German Auslegeschrift 1,080,112 (Ruppert) teaches that trimethylamine oxide dihydrate can be prepared by oxidizing trimethylamine with dilute hydrogen peroxide in the presence of ethyl formate as a catalyst at 30.degree.-40.degree. C., decomposing excess hydrogen peroxide, and concentrating the product solution by vacuum evaporation. However, the mixed tert-amine oxides including at least one long-chain group have not previously been prepared in dihydrate form. It is possible that it was not thought even possible to prepare such dihydrates because of the hygroscopicity that would have been expected to be imparted by the long-chain group or groups.